This invention relates to passively modelocked lasers that are capable of generating subpicosecond pulses. The use of passive modelocking in cw dye lasers to generate subpicosecond optical pulses is now a well-established technique. See, for example, the article entitled "Passive Mode locking of the cw Dye laser," by E. P. Ippen, C. V. Shank and A. Dienes, Applied Physics Letters, Vol. 21, No. 8, Oct. 15, 1972, pp. 348-350. In the Ippen et al laser, a Rhodamine 6G dye cell is placed in a linear laser cavity and pumped by a continuous argon laser. At the other end of the laser cavity is a dye cell containing a solution of DODCI (diethyloxadicarbocyanine iodide) in methanol. This second dye cell acts as a saturable absorber for the Rhodamine 6G emission. Both the active medium cell and the absorber cell are about 1 mm thick in this prior art embodiment and the laser produced a pulse as short as 1.5 picoseconds.
A variation in the linear cavity laser was disclosed by D. J. Bradley in his article in Vol. 18, pp. 53-57 of Topics in Applied Physics entitled "Methods of Generation," Springer-Verlag, 1977. In the Bradley laser, the absorber dye was placed at the extreme end of the linear laser cavity in direct contact with a broadband 100 percent reflectivity mirror and caused to flow in a narrow channel of thickness variable from 200 .mu.m to 500 .mu.m. Bradley and his coworkers noted that a decrease in the thickness of the absorber cell resulted in a decrease of the pulse durations. This technique of placing the absorber cell in direct contact with one mirror of the linear laser cavity was also reported by Bradley and his coworkers in an article entitled "Relationship Between Saturable Absorber Cell Length and Pulse Duration in Passively Mode-Locked Lasers," by D. J. Bradley, G. H. C. New and S. J. Caughey, Optics Communications, Vol. 2, No. 1, 1970, pp. 41-44. As should be apparent to those skilled in the art, there are fabrication problems associated with having the absorber in direct contact with the cavity end mirror. One difficulty is of having the reflecting surface, the absorber, and the focused laser mode all in the same location. Focusing is required to make the absorbing region sufficiently small and this leads to degradation of the absorber on the reflecting surface.
A subsequent article by one of Bradley's coworkers presented an analysis of his linear laser in the IEEE Journal of Quantum Electronics, Vol. QE-10, No. 2, February 1974, pp. 115-124. See the article entitled "Pulse Evolution in Mode-Locked Quasi-Continuous Lasers," by G. H. C. New. In this article by New it is pointed out that single pulse operation is desirable and it is stated that if multiple pulsing develops, it is unlikely that compression occurs for any of the pulses in the cavity. Conclusions of this type were in keeping with similar conclusions developed by those skilled in the art that had been experimenting with ring laser structures. As was well-known in the art, a modelocked ring laser generally had two counterpropagating pulses in the resonant cavity that coincide at the modelocking element. See, for example, the abstract of the talk by R. Fedosejevs et al as reported in the Technical Digest of the 9th International Conference on Quantum Electronics, 1976, pp. 68D-69D. As reported by Fedosejevs et al it was believed by those skilled in the art who were using ring laser cavities that one of the pulses must be suppressed in order to achieve stable trains of pulses with short durations. In the reported talk by Fedosejevs et al, several schemes are described for removing the effect of the second pulse in the ring laser cavity.
The closest that the prior art has come to a position that can be construed as tolerating a second pulse is reported by E. N. Garmire et al in their article entitled "Laser Mode-Locking with Saturable Absorbers," IEEE Journal of Quantum Electronics, Vol. QE-3, No. 6, June 1967, pp. 222-226. In the Garmire article which presented an analysis of modelocking with saturable absorbers in a linear laser, they come to the conclusion that the condition of two traveling pulses at any one time inside the cavity is almost as favorable as that of a single pulse. This conclusion by Garmire et al is based on a first order analysis of the amount of energy absorbed by the dye. As pointed out in the article, "To second order, a single pulse is slightly favored."